1. Field of the Invention
This invention relates generally to a cooling arrangement for an outboard motor, and more particularly to an improved cooling arrangement for an exhaust conduit of an outboard motor.
2. Description of Related Art
An outboard motor typically comprises a housing unit that can be mounted on an associated watercraft. An internal combustion engine is disposed above the housing unit. Typically, the outboard motor employs an exhaust system that includes an exhaust conduit extending generally vertically within the housing unit to discharge exhaust gases from the engine. Because the exhaust gases bear considerable heat, heat is continually transferred to the exhaust conduit during operation of the engine. Cooling the exhaust conduit as well as the engine thus is necessary.
Conventional outboard motors typically employ open-loop cooling systems that draw cooling water from a body of water in which the outboard motor is operated (e.g., a lake or an ocean) primarily to cool the engine. The cooling water also is available for cooling the exhaust conduit and exhaust gases passing therethrough. In some of these systems, the cooling water that has traveled around water jackets in the engine is used for cooling the exhaust system. In other systems, part of fresh water ascending to the engine is delivered to the exhaust conduit.
For example, U. S. Pat. No. 6,027,385 discloses an engine supported by an exhaust guide member that defines a vertically extending exhaust passage communicating with one or more exhaust ports in the engine. A lubricant reservoir depends from the exhaust guide member to store lubricant oil. The lubricant reservoir is affixed to the bottom of the exhaust guide member and forms a central hollow portion. An exhaust conduit is affixed to a portion of the lubricant reservoir and extends through the central hollow portion. The exhaust conduit communicates with the exhaust passage of the exhaust guide member. This patent also discloses a water passage extending through the exhaust guide member and the portion of the lubricant reservoir (FIG. 17). The water passage defines a discharge port underneath the reservoir. The discharge port opens to the central hollow portion. Accordingly, at least part of cooling water can be discharged at a location close to the exhaust conduit.
Japanese Laid Open Publication No. 8-230783 discloses an outboard motor having a water supply pipe that supplies fresh water to the engine. The water supply pipe extends in parallel to the exhaust conduit and defines a small hole facing to the exhaust conduit. Part of the fresh water thus impinges onto an outer surface of the exhaust conduit. In this arrangement, a certain limited area of the exhaust conduit can be cooled. However, a major part of the conduit, particularly an area on an opposite side of the exhaust conduit, is not cooled by the water.